Field of the Invention.
This invention relates generally to chemical solvent cleaning systems and methods and, more particularly, to chemical solvent cleaning systems and methods for use in steam generators used in nuclear power generation to monitor the corrosive effects of chemical solvent cleaning of steam generators.
Prior Art.
Certain nuclear power stations using pressurized water reactor technology utilize a large heat exchanger, known as a steam generator, to transfer heat from the reactor coolant system (i.e. heat generated during nuclear fission) to a secondary system. The heat transfer results in the boiling of secondary steam feedwater, which produces steam to drive the plant's turbine, which in turn powers the electric generator. The steam is then condensed and returned to the steam generator in a continuous recirculation loop. The secondary feedwater contains trace quantities of contaminants, primarily iron oxide, which tend to plate out inside the steam generator, eventually resulting in flow blockage, corrosion, and/or loss of heat transfer capability. An emerging mitigation method for removing such contaminants involves the use of a chemical solvent, usually during unit shutdown, to dissolve the contaminants.
The thermal efficiencies of steam generators are reduced due to accumulations of iron and copper corrosion deposits. In actual operation at Arkansas Nuclear One--Unit 1 (ANO-1) at Russelville, Ark., it was observed that such deposits occurred primarily between the fourth and ninth tube support plates of the once through steam generators (OTSG's) at ANO-1. Highly effective, low corrosive solvents for removing iron and copper deposits have been developed through a program sponsored by the Steam Generator Owners Group (SGOG), and the Electric Power Research Institute (EPRI).
Prior art chemical cleanings of OTSG's utilize a process known as a "fill, soak and drain" (F/S/D) process. Some recirculation-type steam generators are cleaned using a circulation method, but only to clean the very lowest parts of the generator. F/S/D was developed to assure adequate mixing of solvent between the inner tube bundle of an OTSG and the annulus region (between the outer shell and inner shroud of the generator). This method, while effective for mixing, increased the overall time and thus the cost of the cleaning process by requiring additional steps to be performed throughout the cleaning process. Further, temperature differentials in the solvent between the top and bottom of the steam generator, as well as solvent stagnation in the annulus region of the generator created concerns about the effectiveness of F/S/D. F/S/D requires additional intermediate heating steps due to the time required to empty the generator. These intermediate steps further increased the overall time for the cleaning process as well as the amount of waste generated by the process. Further, F/S/D methods often use a high initial solvent temperature to offset cooling which takes place during the process, which can result in increased corrosion of internal steam generator components. A cleaning system and method was needed to assure adequate mixing of fresh solvent throughout the relevant portions of the steam generator.